Near-Surface Cobalt Implantation Into Amorphous Carbon Films: Observation Of Complex Magnetic Nanostructures And Multiple Magnetic Phases

Magnetic nanoclusters in amorphous carbon have promising applications for highly responsive magnetic sensors, where decreasing the size of the nanoclusters can lead to superparamagnetism and therefore low remanence. The insulating properties (wide bandgap) of amorphous carbon are also potentially useful for designing high frequency components. Both these properties are crucial to achieve ultra-high density magnetic data storage.

High fluence (1.2×10¹⁷ Co/cm²) near-surface implantation of 30 keV Co ions into amorphous carbon results in the formation of complex magnetic nanostructures and multiple magnetic phases. Next to small segregated cobalt carbide nanoclusters, starting forming at a depth of 25 nm within the amorphous carbon film, a nearly continuous network of cobalt carbide thin nanocrystalline regions can be observed at a depth of 8 nm. On the surface a 3 nm thin cobalt oxide nanostructured layer is seen separated from the cobalt carbide by a 1 nm thin Co-depleted region. TEM and magnetic measurements show superparamagnetic nanoclusters with a blocking temperature of 5 K. However, a small proportion of larger cobalt carbide nanoclusters exhibits magnetic hysteresis even at room-temperature. The magnetic saturation moment is as high as 0.51 µB/Co at 2 K and 0.32 µB/Co at room temperature - ten times larger than previously reported on hydrogenated amorphous carbon [1]. The structural disorder of the nanoparticles results in a spin glass behaviour with a range of transition temperatures below ~70 K, suggesting a spin disordered shell model [2]. Thus high fluence Co-implantation into amorphous carbon at room temperature created complex magnetic nanostructures consisting of cobalt oxide and cobalt carbide. Multiple magnetic phases such as superparamagnetism, spin glass, ferromagnetism and also antiferromagnetism can be observed.

References:

1. P.G. Sridhar Gupta, G.V.M. Williams and A. Markwitz, Journal of Physics D: Applied Physics, 2016, 49, 5, 055002.
2. T. Prakash, G.V.M. Williams, J. Kennedy and S. Rubanov, Materials Research Express, 2016, 3, 12, 126102.